The postnatal development of adrenergic receptor mechanisms will be studied in newborn piglets of various ages. Cardiovascular responses to adrenergic receptor-blocking agents, and to adrenergic stimuli before and after blockade of receptor systems, will be recorded in intact animals and isolated heart preparations. Adrenergic stimuli will include catecholamine infusions, efferent sympathetic nerve stimulation, and peripheral baroreceptor and chemoreceptor reflex activation. The cardiovascular responses selected will include heart rate, arterial and ventricular pressures, and regional blood flows (femoral, renal, cephalic and coronary). The latter will be measured by electromagnetic flowmeters. After a control period of one or more types of adrenergic stimulation, the effective blocking dose of a particular agent (phentolamine, phenoxybenzamine, propranolol or sotalol, and proctalol) will be determined; then the same types of adrenergic stimulation will be repeated. The existence of age-dependence for minimal effective receptor-blocking dose, and for any difference in degree of cardiovascular response before and after receptor blockade, will be established statistically.